Lowpass filter cutoff is the –3 dB cutoff
frequency for the single-pole lowpass filter at the front of the
reverberator structure. It prevents the application of reverberation to
high-frequency components of the input.

To specify Highcut frequency (Hz) from an input port,
select Specify from input port for the
parameter.

Diffusion is proportional to the rate at which the
reverb tail builds in density. Increasing Diffusion
pushes the reflections closer together, thickening the sound. Reducing
Diffusion creates more discrete echoes.

To specify Diffusion from an input port, select
Specify from input port for the parameter.

Decay factor is proportional to the time it takes for
reflections to run out of energy. To model a large room, use a long reverb
tail (low decay factor). To model a small room, use a short reverb tail
(high decay factor).

To specify Decay factor from an input port, select
Specify from input port for the parameter.

High frequency damping is proportional to the
attenuation of high frequencies in the reverberation output. Setting
High frequency damping to a large value makes
high-frequency reflections decay faster than low-frequency
reflections.

To specify High frequency damping from an input port,
select Specify from input port for the
parameter.

Dependencies

Interpreted execution – Simulate the
model using the MATLAB® interpreter. This option reduces startup time and the
simulation speed is comparable to Code
generation. In this mode, you can debug the source
code of the block.

Code generation – Simulate the model
using generated C code. The first time you run a simulation,
Simulink® generates C code for the block. The C code is reused
for subsequent simulations, as long as the model does not change.
This option requires additional startup time, but the speed of the
subsequent simulations is comparable to Interpreted
execution.

Block Characteristics

Algorithms

The algorithm to add reverberation follows the plate-class reverberation
topology described in [1] and is based on
a 29,761 Hz sample rate.

The algorithm has five stages.

The description for the
algorithm that follows is for a stereo input. A mono input is a simplified
case.

Stereo-to-Mono

A stereo signal is converted to a mono signal: x[n]=0.5×(xR[n]+xL[n]).

Preconditioning

A delay followed by a lowpass filter preconditions the mono
signal.

The pre-delay output is determined as xp[n]=x[n−k], where the Pre-delay
(s) parameter determines the value of k.

The signal is fed through a single-pole lowpass filter
with transfer function

LP(z)=1−α1−αz−1,

where

α=exp(−2π×fcfs).

fc is the
cutoff frequency specified by the Pre-delay (s) parameter.

fs is the
sampling frequency specified by the Inherit sample rate
from input parameter or the Input sample rate
(Hz) parameter.

Decorrelation

The signal is decorrelated by passing through a series of four
allpass filters.

The allpass filters are of the form

AP(z)=β+z−k1+βz−k,

where β is the coefficient specified
by the Diffusion property and k is
the delay as follows:

For AP1, k =
142.

For AP2, k =
107.

For AP3, k =
379.

For AP4, k =
277.

Tank

The signal is fed into the tank, where it circulates to simulate
the decay of a reverberation tail.

The following description tracks the signal as it progresses
through the top of the tank. The signal progression through the bottom
of the tank follows the same pattern, with different delay specifications.

The new signal enters the top of the tank and is added
to the circulated signal from the bottom of the tank.

The signal passes through a modulated allpass filter:

ModulatedAP1(z)=−β+z−k1−βz−k

β is the coefficient specified
by the Diffusion parameter.

k is the variable delay specified
by a 1 Hz sinusoid with amplitude = (8/29761) × (sample rate).
To account for fractional delay resulting from the modulating k,
allpass interpolation is used [2].

The signal is delayed again, and then passes through
a lowpass filter:

LP2(z)=1−φ1−φz−1

φ is the coefficient specified
by the High frequency damping parameter.

The signal is multiplied by a gain specified by the Decay
factor parameter. The signal then passes through an allpass
filter:

AP5(z)=β+z−k1+βz−k.

β is the coefficient specified
by the Diffusion parameter.

k is set to 1800 for
the top of the tank and 2656 for the bottom of
the tank.

The signal is delayed again and then circulated to
the bottom half of the tank for the next iteration.

A similar pattern is executed in parallel for the bottom half
of the tank. The output of the tank is calculated as the signed sum
of delay lines picked off at various points from the tank. The summed
output is multiplied by 0.6.

Wet/Dry Mix

The wet (processed) signal is then added to the dry (original)
signal:

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